Despite recent epidemic proportions of dietary obesity, there is a considerable lack of knowledge about the neurochemical effects of obesity on those midbrain dopamine projections involved in motivation and food reward. A fundamental issue is whether the effects of dietary obesity on CNS function are due to changes in body weight and bodily metabolism;or due to the palatability of high-energy diets, which could significantly alter synaptic plasticity in midbrain dopamine systems and change their response to dopamine-releasing agents, such as food or drugs of abuse. In this proposal we will consider the hypothesis that chronic exposure to a cafeteria-type palatable diet (leading to excessive weight gain) significantly reduces basal and evoked dopamine release in the adult rat nucleus accumbens and other targets of the midbrain dopamine cell bodies (such as the medial prefrontal cortex and the striatum). We also predict that this attenuation in basal and evoked dopamine release would reduce the dopamine-releasing efficacy of palatable meals and induce obese animals to increase the intake of palatable food or other non-physiological dopamine-releasing reinforcers (like psychostimulants) in order to achieve dopamine release comparable to that of a normal weight animal. Furthermore, we predict that an increase in evoked dopamine signal in central dopamine systems early in postnatal age is a marker for obesity resistance. Our specific objectives are as follows: A. Identify selective effects of dietary obesity (induced by a cafeteria-type diet versus a high-fat diet versus high-carbohydrate diet) on basal, meal- and psychostimulant-challenged dopamine neurochemistry in all major pathways of the adult rat midbrain dopamine system (mesoaccumbens, nigrostriatal, corticolimbic) with in vivo microdialysis. Correlate changes in dopamine release with changes in behavior (locomotion, diet consumption, response to injections of the opiate antagonist naloxone). Compare results with those on rats inbred for several generations to be either obesity-prone or obesity-resistant. B. Identify selective effects of dietary obesity on electrical stimulation-evoked dopamine release in real time in all major pathways of the adult rat midbrain dopamine system (mesoaccumbens, nigrostriatal, corticolimbic) with acute slice amperometry. Study differential effects of obesity on neurotransmitter release versus reuptake. Compare slices from normal weight, dietary obese, inbred obesity-prone and inbred obesity-resistant animals. C. Use cultured ventral tegmental area dopamine neurons from inbred dietary obesity-prone and obesity-resistant neonatal rats to consider whether dopamine quantal release is higher in obesity-resistant animals before actual exposure to palatable diets and excessive weight gain